With the recent remarkable progress of information society, an increasing number of information communication apparatuses and services employ wireless communication in addition to wire communication as a communication method. This boosts the demand for radio frequencies that are a finite resource. Consequently, all nations around the world are faced with the problem in that frequencies to be allocated may be depleted. In general, each individual nation controls frequency usages by assigning licenses, and only licensed persons are permitted to use predetermined frequencies at specific places and times under strict control. In order to keep up with currently growing frequency demand, however, there is a need to conceive of a novel method of using frequencies which is free of traditional methods.
In recent years, in order to solve the problem of depletion of the frequencies, studies have been conducted on methods of using frequency bands (white spaces) that have already been allocated but not been used spatially and temporally. To give an example, cognitive wireless communication systems have been studied. Such systems permit non-licensed users (referred to below as “secondary users”) to use white spaces flexibly while sufficiently avoiding affecting incumbent systems for licensed users (referred to below as “primary users”) which are operating at certain frequencies (e.g., refer to Non Patent Literature 1).
IEEE (Institute of Electrical and Electronics Engineers) 802.22 standardizes a wireless communication system (wireless regional area network (WRAN)) using white spaces. In this system, a wireless station accesses a database (DB) over an IP network, and acquires an available channel list (frequency list) and maximum permitted transmitting power on the basis of its location information. The available channel list is continuously updated and exclusively managed by a spectrum manager (SM) within a base station (BS). The BS decides on channels available for communications through the available channel list, and sets these channels as operational channels. Then, the base station BS communicates with terminals (customer premises equipment (CPE)), such as mobile units and portable phones, installed individually within the coverage area (cell) of the base station BS.
The SM also manages spectrum sensing information. Wireless stations, such as the BS and the CPE, may each be equipped with a spectrum sensing function. When each wireless station detects that the operational channel is used by an incumbent system (primary user's system) through the spectrum sensing function, it notifies the SM of information regarding the detection. Then, the SM notifies the DB of the detected information, and deletes this operational channel from the available channel list. In this way, the wireless communication system using white spaces makes dynamic spectrum accesses on the basis of continuously updated information, thereby enabling secondary users to conduct communications while avoiding affecting the channels used by the primary users.
According to IEEE 802.22, both a wireless communication system using white spaces and a system for a primary user's system (e.g., digital television (DTV)) use the same frequency band. Accordingly, the wireless communication system using white spaces needs to avoid affecting primary users' systems. For that purpose, it is necessary to restrict the transmission power. According to IEEE 802.22, for example, if a wireless communication system using white spaces is a fixed wireless station, the maximum transmission power is 4 W; if it is a portable wireless station, the maximum transmission power is 100 mW. Because of the limitation on the transmission power, a wireless station using white spaces receives relatively strong interference from a DTV system, which is problematic to its operation.
An exemplary wireless system known in the art is designed to easily avoid interference caused by the communication between a plurality wireless base stations. This wireless system includes a plurality of wireless base stations that conduct wireless communication with one another. Each wireless base station has an adaptive array antenna constituted of a plurality of antenna elements. The wireless system stores link identification data that is assigned uniquely to a linear propagation path for communication between the wireless system and a partner wireless base station, in relation to another wireless base station that acts as an interference source during this communication. When the wireless system receives the link identification data from the partner wireless base station, it performs null control on the basis of the link identification data in such a way that the null point is oriented to the wireless base station acting as an interference source (e.g., refer to Patent Literature 1).